The invention pertains to procedures for joining stainless steel or superalloy parts by diffusion brazing and to the interface materials used in such joining procedures intended to form assemblies.
The term "superalloy" is understood to mean all materials which exhibit very great strength even at high temperatures and whose typical characteristic is that they basically consist of a solid nickel-chromium or cobalt-chromium solution, with the addition of aluminum, titanium and/or refractory materials to strengthen the solution and of carbon, boron (and/or) zirconium to promote ductility in creep rupture.
The term "interface material" refers to the filling material placed between the surfaces to be joined together. It consists of a layer interposed between the contacting surfaces of the parts to be joined and, more specifically, is in the form of a plating, a strip, or a plating-strip combination.
Diffusion welding in the solid state, under high pressure and heat, constitutes an attractive approach to the manufacture of parts having exact dimensions and high mechanical resistance to heat for use in aircraft engines. In particular, the applied pressure causes an intimate contact of the surfaces, which is necessary for the diffusion of the constitutents of the facing surface layers. In other words, if the joined parts are identical in composition and microstructure, it should be impossible to distinguish the interface from the other regions of the assembly by any physical, physico-chemical or mechanical test or examination.
Unfortunately, in addition to the difficulties encountered in obtaining suitable surface conditions to ensure perfect surface-to-surface contact so as to allow the solid state transport phenomenon to take place uniformly over the entire joint, the temperature rise preceding the welding operation causes certain constituents to migrate toward and become segrated in the surface layers of the parts, possibly hindering or even preventing the diffusion. The formation of such a diffusion barrier occurs particularly in superalloys containing, besides a non-negligible proportion of carbon, a relatively high titanium content (say, over 0.5%). Following diffusion welding, it can be observed, first, that the interface contains a nearly continuous border formed by segregated titanium compounds and consisting primarily of titanium carbide or carbonitride, and, second, that there is an absence of recrystallization. Mechanical tests confirm that joints produced in this manner are defective.
Previously proposed remedies associate the techniques of brazing, which is less sensitive to the precision of contact between the surfaces to be joined together, and of diffusion welding, in procedure known as "diffusion brazing".
A first category of diffusion brazing procedures calls for inserting, between the two parts to be joined together, a thin layer (preferably in powder form and held in a pyrolyzable organic binding material) of an alloy having the same basic components as the superalloy of the parts but also containing additives (fluxes) such that the liquidus temperature of said alloy is lower than the initial fusion temperature of the superalloy. The heating initially causes this layer to fuse and to become joined with the contacting surface layers of the parts, and then causes the local content of said flux (or fluxes) to diminish as a result of its migration in the neighboring regions of the parts.
In a second category of known procedures--which may be used in combination with the first--an intermediate layer of the base metal or metals of the superalloy, whose composition is such that it does not include any elements which produce structural hardening (such as titanium or aluminum), is formed in advance between the two parts. The bonding occurs by interdiffusion between the strip and the surface regions of the parts or by interdiffusion of the plated layers. This is followed by a homogenizing heat treatment. As an example of known procedures, see British Pat. No. 1,430,587.